Polymers containing keto-groups are in general high performance engineering thermoplastics, often having the advantages of chemical resistance, good high temperature properties, good tensile properties and others. They are commercially significant because of their good mechanical properties and thermooxidative stability. There are two broad classes of keto group containing polymers, namely aliphatic polyketones and aromatic poly(ether ketone)s (cf. FIGS. 1 and 2).
Aliphatic polyketones are prepared by the co-polymerization of olefin(s) and carbon monoxide (see FIG. 1). In this case the co-polymerization may proceed, for example, free radically or as catalyzed by Palladium metal by “insertion” polymerization.
Aromatic poly(ether ketone)s are obtained by electrophilic Friedel-Crafts acylation polymerization (cf. FIG. 2B) or by aromatic nucleophilic substitution (cf. FIG. 2C). The nucleophilic route typically employs a metal salt of a difunctional phenolic compound and a dihalobenzophenone along with a base in high boiling dipolar aprotic solvents or sulfolane at high temperatures.
There are a few drawbacks associated with this process. The halo benzophenone monomers employed in the polymerization are predominantly derived from fluorobenzophenone and these monomers are very expensive. Also these reactions produce by-products like inorganic fluorides which must be properly disposed of. In the electrophilic substitution route an acid halide of a difunctional carboxylic acid is complexed with aluminum trichloride which is then reacted with aromatic compounds activated for electrophilic substitution. Because of the fact that this reaction proceeds heterogeneously, the molecular weights of the polymers produced are generally undesirably low. Also, since excess aluminum trichloride has to be employed, which later must be separated from the polymer and as a result generates a lot of inorganic waste in the process. Furthermore, the acid halide employed is also highly moisture sensitive and the presence of traces of water can convert it back to the unreactive carboxylic acid which would create stoichiometric imbalance. As these are step growth polymerizations, any change in stoichiometry would ultimately result in unfavourable low molecular weight materials. An alternative process employs very highly corrosive acid mixtures like boron trifluoride-hydrofluoric acid.
Alternative condensing agents for making keto groups through aromatic electrophilic substitution are also available. For example, a system consisting of phosphorus pentoxide/methane sulfonic acid in a weight to volume ratio of 1:10 has been used as condensing agent in the preparation of poly(ketone)s. (cf. Ueda, M., et al., “Synthesis of aromatic poly(ether ketone)s in phosphorus pentoxide/methane sulfonic acid”, Polymer Journal, Vol. 21, No. 9, pp 673–679 (1989); Ueda, M., et al., “Synthesis of polyketones by direct polycondensation of dicarboxylic acids with diaryl compounds using phosphorus pentoxide/methane sulfonic acid as condensing agent and solvent”, Makromol. Chem., Rapid Commun. 5, pp 833–836 (1985); Ueda, M., et al., “Synthesis of aromatic poly(ether ketone)s”, Macromolecules, 1987, 20, pp 2675–2678; Parthiban, A., et al., “Preparation and Characterization of co- and homo-poly(phenylene etherimide ketone)s”, J. Polym. Sci.: Part A: Polym. Chem., Vol. 31, pp 1233–1241 (1993)). Many other modifications of this condensing agent are available.
For example U.S. Pat. No. 4,820,792 discloses the use of fluoroalkane sulfonic acid or methane sulfonic acid along with other acids like polyphosphoric acid, trichloroacetic acid and trifluoroacetic acid. There are several disadvantages associated with this process. Polyphosphoric acid is highly viscous and hence it is difficult to handle. The weaker acids also require the use of highly corrosive stronger acids and thus these acids would pose a lot of problems in terms of plant maintenance. Because of the reduced acidity of the condensing agent, a very long reaction time is required in order to get polymers with appreciable viscosities. In addition to these drawbacks, the process is cumbersome as it involves various heating stages which contributes to higher energy costs and the formation of undesired structures. Also the use of halogenated sulfonic acids like trifluoromethane sulfonic acid or trifluoroacetic acid is not economically advantageous.
U.S. Pat. No. 5,155,203 discloses the preparation of poly(ether ketone) using phosphorus pentoxide/methane sulfonic acid as condensing agents. However, it is a co-polymer made up of two different difunctional monomers namely 1,3-bis(4-phenoxy benzoyl) benzene and 4,4′-oxydibenzoic acid. The carboxylic acid employed is aromatic in nature. As a result, diary ketone is formed which, however, is unsuitable for further organic reactions.
UK patent application No. GB 2,355,464 discloses the preparation of poly(ether ketone)s using alkyl or aryl sulfonic acids with the absence of phosphorus pentoxide so that no neutralisation step is required after the polycondensation reaction. In place of phosphorus pentoxide higher temperatures are employed. The water formed as a result of polycondensation is removed either by passing inert gas like nitrogen into the reaction mixture which drives off the water in the form of its vapour or by removing water as an azeotrope with solvents like xylene or toluene. One of the drawbacks of this process is that the solvents toluene or xylene used for forming the azeotrope can act as end capping agents and cause premature termination of the growing polymer chain. A second drawback is that aromatic carboxylic acids are employed and hence the process results in the formation of diary ketones which is not useful for further modification of the polymer structure. Also higher temperatures will induce the formation of undesired structures.
U.S. Pat. No. 6,538,098 discloses a process for the polymerization of dicarboxylic acid with an electron rich bi-reactive compound using a combination of phosphoric acid and trifluoroacetic anhydride. Again aromatic dicarboxylic acids are employed yielding diaryl ketones which are unsuitable for further modifications. Also the use of trifluoro acetic anhydride would make this process uneconomical.
U.S. Pat. No. 6,566,484 B2 discloses the preparation of melt processible poly(ether ether ketone) (PEEK) from phenoxy phenoxy benzoic acid using methane sulfonic acid containing methane sulfonic anhydride or phosphorus pentoxide. Yet again this process generates unmodifiable diaryl ketones, too.
All above-mentioned processes lead to unfunctionalized polymeric diaryl ketones which are difficult to modify. However, modification of these polymers is necessary in order to improve some or many of the properties of the base polymer. In general, there are two ways to functionalize polymers. One is the use of a functionalized co-monomer during the co-polymerization. For example, Parthiban et al. (“Amino-functionalised poly(arylene ether ketone)s”, Macromolecules, 1997, Vol. 30, pp 2238–2243), have reported the preparation of amine functionalized polyarylene ether ketones using this approach. The disadvantage of this approach is that the functionalized monomers, in a majority of cases, are not readily available and hence the applicability of this approach is very limited.
The second route involves a modification reaction on the preformed polymer. This approach has many setbacks. The type of reactions that can be carried out on the preformed polymer is very limited. Also the functionalization is difficult to control in such cases. Since there are many reactive sites even within the repeating unit, most often these reactions are non-uniform.
U.S. Pat. No. 5,288,834 discloses the preparation of functionalized polyarylene ether ketones through the aromatic nucleophilic substitution route. Methyl group substituted hydroquinone has been used as co-monomer. In the subsequent steps the methyl group was activated by brominating the methyl group to yield various derivatives of polyarylene ether ketones. The disadvantages discussed above for the nucleophilic substitution are also applicable for this process. In addition the use of substituted bifunctional phenolic compounds make the process even more uneconomical.
U.S. Pat. Nos. 5,344,914 and 5,442,029 disclose the preparation of various poly(ketone)s via the nucleophilic substitution method. In this process aminonitriles are employed as nucleophiles. There are many drawbacks associated with this process. The preparation of the nucleophilic agents aminonitriles is an unsafe process. It involves the conversion of aldehydes into aminonitriles by reacting the aldehyde with a secondary amine and sodium cyanide. It is well known in the art that sodium cyanide is a highly poisonous compound and the use of it in equimolar quantities would make the process extremely unsafe. The generation of anion, needed for nucleophilic attack, also requires bases like sodium hydride which is pyrophoric in nature and hence has to be handled with extreme care. The process is highly unfavourable in terms of atom efficiency as well. For example the generation of each oxygen atom (atomic weight=16) is accompanied by the loss of much larger units like secondary amine and cyano groups. After acid hydrolysis the presence of cyanide ion even in traces causes a lot of problems for waste treatment and disposal. In addition the use of costly fluorobenzophenones in the preparation of poly(aryl ketone)s increases the cost of the overall process.
All the abovementioned processes result in difficult to modify diaryl ketones and are cited herein only as reference and are not admitted to be prior art.
Thus it is clear from the foregoing examples that the polymers as well as the respective synthetic techniques, which are employed currently, are limited in scope and applicability. Hence it would be desirable to develop a process leading to novel polymeric ketones which are functionalized in themselves, i.e. backbone functionalized, thereby aiding further modifications resulting in various derivatives which are industrially important, novel materials.